1. Field of the Invention
The present invention relates to a siamese-type cylinder block blank and more particularly, to such a cylinder block blank in which a sleeve made of a cast iron is cast, upon the pouring of a molten metal under a pressure, in each cylinder barrel of a siamese-type cylinder barrel made of an aluminum alloy and consisting of a plurality of cylinder barrels connected in series, and an apparatus for casting the same.
2. Description of the Prior Art
In conventional siamese-type cylinder block blank, each sleeve presents a substantially oval configuration in section with the lengthwise axis perpendicular to the direction of cylinder barrels arranged because the opposed peripheral walls of the adjacent sleeves are strongly subjected to the pouring pressure of a molten metal during pouring of the latter under a pressure into a mold.
In this case, the configuration in section of each cylinder barrel at the solidification shrinkage thereof is substantially oval with the lengthwise axis parallel to the direction of cylinder barrels arranged and hence, each sleeve is subjected to the solidification shrinkage force of the aluminum alloy and intended to be deformed to follow the configuration in section of each cylinder barrel at its shrinkage, but the sleeve deformed is changed in the configuration at the pouring of molten metal to a slight extent.
This results in the configurations in section of each sleeve and barrel with their lengthwise axes offset approximately 90.degree. from each other, causing the casting stress remaining in each sleeve to be ununiform around its inner peripheral surface. When the sleeve as it is in such a state is subjected to a working for its inner peripheral surface into a true circle to give a cylinder block, and this block is used to assemble an engine, the operation of the latter causes the amount of resulting sleeve thermally expanded to be ununiform around its circumference. For this reason, a clearance may be produced between a piston ring and the sleeve, resulting in an increased amount of blow-by gas and in a useless consumption of oil.
In addition, in the conventional cylinder blocks, the sleeve as cast has been cast in each cylinder barrel. On the outer peripheral surface of each sleeve, annular or spiral slip-off preventing grooves have been made at a predetermined pitch during the casting of the sleeve by the mold to extend in the circumferential direction over a predetermined length from the sleeve end to which a cylinder head is bound. The slip-off preventing groove is generally U-shaped in cross section.
However, the use of the sleeve as cast causes the close adhesion between the molten metal and the sleeve to be hindered because of the microporosity of the outer peripheral surface of such sleeve and thus, a very small clearance may be produced between the sleeve and the cylinder barrel. If the slip-off preventing groove is made into a U-shape in cross section, then a gas such as air is settled at the corners between the inner side and bottom surfaces of the groove during casting and is confined therein by the molten metal. This also causes a very small clearance to be produced between the sleeve and the cylinder barrel as described above. In a siamese-type cylinder block, the ajacent sleeves are very close to each other, and between these sleeves there is generally no portion of a water-jacket. Therefore, the heat at the portions of both the sleeves opposed to each other may be transferred in a shortest path to the water jacket through the barrel located between these sleeves, but if a very small clearance as described above is produced around the outer periphery at those portions of both the sleeves opposed to each other, such heat transfer path is disconnected, causing the release of heat of the sleeve not to be effected uniformly around its circumference. Thus, the efficiency in release of heat of the sleeve is reduced.
The shaping of individual slip-off preventing grooves by the mold results in a wide variation in depth thereof and in an unevenness in thickness of the sleeve at the slip-off preventing grooves and the land portions between the adjacent grooves.
In such a cylinder block, the amount of sleeve expanded is unniform around the circumference of the sleeve and hence, the same problems may arise as described above.